?? T cells constitute an important component of the immune response against infectious agents, cancer and auto-immunity yet the biochemical mechanisms by which they detect antigen through their T cell receptor (TCR) remain unclear. In humans, in particular, the nature of ?? T cell ligands remains ambiguous, as is the role of?? T cells in health and disease. Thus, the long-term goal of this proposal is to understand the functional, biochemical and structural mechanisms of ?? TCR ligand engagement and how this binding event discriminates between healthy and unhealthy tissue. We made considerable progress in our initial funding period, with five total publications resulting from our work, published in well-recognized journals (Nat. Immunol., Immunity, PNAS, JBC and EMBO). Our current proposal seeks to extend our work on ?? T cell recognition to a newly discovered human interaction, that of the lipid presenting, MHC-like molecule CD1d. In collaboration with Prof. Albert Bendelac, we have been characterizing a population of human V?1+ ?? T cells that respond to CD1d presenting the lipid sulfatide. Based on our preliminary data, we propose three aims focused on unraveling the function of this interaction in human immunity and disease. Aim 1: Characterization of a human ?? T cell population specific for CD1 molecules., seeks to expand our limited knowledge of the identity of this population. We will investigate the frequency of these cells in a diverse donor pool and characterize them both molecularly and functionally. This aim will also focus on the potential for these cells to be cross-reactive with other lipid antigens to identify whether molecular mimicry plays a role in their reactivity. Aim 2, Elucidatio of the molecular mechanisms by which ?? TCRs bind to CD1d/lipids., will focus on characterizing the interaction between the ?? TCRs expressed by these cells and CD1d/sulfatide. We will use protein biochemistry, biophysics and x-ray crystallography to elucidate the molecular mechanisms by which the ?? TCR recognizes CD1d/sulfatide. This will represent one of the few molecularly characterized ?? TCR/ligand interactions and will significantly advance our understanding of ?? T cell recognition. We will also biochemically screen for other CD1d/sulfatide specific ?? T cells in disease, i.e. multiple sclerosis., will expore the role of these sulfatide-reactive cells in the highly debilitating auto-immune disease, MS. Because V?1+ ?? T cells are found at high frequencies within multiple sclerotic plaques, we seek to investigate whether sulfatide reactivity may be playing a role in the initiation or progression of the disease state. ?? T cells can be either pro-inflammatory or regulatory, therefore we seek to understand which role these cells play, if any, in MS. Together, these aims will begin to unravel the mystery of ?? T cells in human immunobiology, both at the cellular and molecular levels. Because so little is known about these cells, the studies proposed here will fundamentally advance our understanding of what these cells see and, ultimately what these cells do in the human immune response. lipid antigens, directly complementary to our lipid screen in Aim 1. Aim 3, Determine the role of